1. Field of the Invention
This invention relates generally to servo indicators and, more particularly, to digital servo indicators using the continuous null-balance potentiometric principle to measure and balance unknown signals in the submicrovolt range and produce a digital readout in the desired engineering unit.
2. Description of the Prior Art
Prior art servo indicators have used the null-balance potentiometric principle to balance unknown voltages and to provide a digital readout in the desired engineering values or units. In some of these prior art devices, the balance was approached linearly at a single rate, in which instance the damping was often ineffective and the null or balance point was frequently overshot, with the result that balance must then be approached from the other direction, also at a single rate of correction, until, like a pendulum coming to rest, the null point was reached.
Some prior art devices have attempted to overcome the problem of overshooting by incorporating two linear rates of correction--a faster rate when the error signal was large, and a slower, but likewise linear, rate of correction when the error signal was at or less than a predetermined limit.
In U.S. Pat. No. 3,577,075, issued to Krygeris, an indicator was disclosed which had a narrow bandwidth at or near the balance which prevented overshooting of the balance position and which controlled and provided a rate of error correction which was a function of the magnitude of the error signal, so that the approach to the balance was at a progressively slower rate, essentially asymptotic to the balance, and the balance was therefore approached rapidly when the error signal was large but slowly when the error signal was small. This indicator represented a significant improvement over the prior art. However, the Krygeris indicator had several shortcomings. Two digital-to-analog converters were used, each of the Kelvin-Varley type. In addition, the BCD output had limited utility and was not able to provide the versitility of modern microcomputers.